Awaiting an outcome, or termination, of something with almost unbearable anticipation (as in clenching one's fists tightly to get through the agonizing final days or moments of waiting for something to happen or be over with)

Brooke's mom was told she couldn't have a drink for 30 days before the wedding, and she knew she would have to white-knuckle it!

When someone sucks at everything but can't admit it to themselves and get angry at everyone trying to ruin specific peoples lives but fails at it. Especially when it comes to sex they are ALWAYS white knuckled

See that girl?
Yea
She's a white knuckle, don't talk to her! she'll make you regret it!